The present invention relates to maintenance of helicopters in general and to adjusting trim tabs of helicopter rotor blades in particular.
Helicopter rotor blades vary in thickness from leading edge to trailing edge. Their trailing edges are tapered to be very thin compared to the balance of the blade, defining what are called xe2x80x9ctrim tabs.xe2x80x9d Trim tabs are designed to be bent slightly up or down to reduce low frequency vibrations that would otherwise occur when the rotor blades are rotating. Adjusting the degree of bend in the trim tabs is part of routine helicopter maintenance. Instruments in the helicopter provide information about which rotor blades and which parts of each blade need to be adjusted and the amount of adjustment needed.
There are few devices available to adjust trim tabs, such as those described in U.S. Pat. No. 5,111,676 issued to Ruzicka Jr. et al, and U.S. Pat. No. 2,422,042 issued to Roberts. The device used most often requires considerable time to set up and use. Therefore, there remains a need for an improved system for adjusting trim tabs of helicopter rotor blades.
According to its major aspects and briefly recited, the present invention is a system and method for adjusting helicopter rotor blade trim tabs. The system permits accurate bending of the tabs.
In particular, the system includes the following major components: a gage with a folding arm, a bender, and a gage calibration plate. The gage arm is folded and then clamped to the gage calibration plate for calibration. The gage calibration plate is formed to simulate a rotor blade with trim tabs oriented at zero degrees, so that the gage, when clamped in place to it, can be zeroed. Once calibrated, the gage is removed from the plate, unfolded, and then placed on the rotor blade. Unlike prior art trim tab bending systems, the present gage is designed to roll freely along the length of the blade so that the operator can easily position it where it is needed. Once the gage is in position, the bender is applied to the trim tabs at that particular location. The gage will indicate to the operator the precise angle at which the tabs are bent. By grasping the bender handle, the operator can bend the trim tabs up or down while observing the degree of bend on the gage. When the trim tabs have been adjusted to the desired angle, the bender is removed and the gage rolled to a different position on the rotor blade for adjustment at the new position.
An important feature of the present invention is the capability of the gage to roll freely along the length of the rotor blade. This feature has the corresponding advantage of allowing the user to position and reposition the gage quickly and easily at any position along the rotor blade.
Another important feature of the present invention is the method of calibration of the gage. This method is simple enough to do prior to adjustment of each blade. Simply by placing the folded gage on the gage calibration plate, locking the gage in place, dropping the depth rod to the plate, and then rotating the gage dial to zero it, the gage is calibrated.
Still another feature of the present invention is the use of a gage that measures bend to xc2xd of a degree. This allows very accurate adjustments.
Related to this feature is the use of manual bending. With a little experience and keeping an eye on the gage indicator, an operator can quickly develop the skill to accurately adjust the trim tabs using a simple bender.